SYSTEM IDENTIFICATION AND RESPONSE ANALYSIS OF RC HIGH-RISE BUILDINGS UNDER SUCCESSIVE EARTHQUAKES

Transcription

1 SYSTEM IDENTIFICATION AND RESPONSE ANALYSIS OF RC HIGH-RISE BUILDINGS UNDER SUCCESSIVE EARTHQUAKES Muhammad Rusli MEE54 Supervisor: Taiki SAITO ABSTRACT System identification was performed to two RC high-rise buildings located in Tokyo, Japan, one was 37 stories and the other was 3 stories. Response analysis was conducted to the 37-story building for which there was structural data available. The strong motion data of the buildings that recorded the successive earthquakes: before, during, and after the mainshock of the Off the Pacific Coast of Tohoku Earthquake were used as input in system identification to obtain dynamic parameters. The parameters as results of system identification were then used as input data in response analysis to observe the behavior of the MDOF model under some earthquake motions and to determine the adequate damping type that governs the behavior. Health monitoring was conducted as a combination of system identification and response analysis to monitor the more specific part of the building. The dynamic parameters of the buildings after the mainshock were changed compared to that before the mainshock, the frequency decreased to 4%, the period increased to 3%, and the damping factor increased to 5%. The adequate damping types of the MDOF model before the mainshock were modal damping and Rayleigh damping, while that during the mainshock were proportional damping to initial stiffness and proportional damping to nonlinear stiffness. The structural members were in a good condition before the mainshock, but the stiffness degradation occurred when they were struck by the mainshock, the story shear force exceeded its cracking capacity but still below the yielding capacity, with maximum ductility.4, it happened to almost the entire structure except the 4 top stories. The decreases of frequency on the upper and lower part of the building were 5% and 8% respectively that indicates the stiffness degradation on the lower part was higher than that on the upper part. Keywords: Reinforced Concrete, High-rise, Earthquake, System Identification, Response Analysis.. INTRODUCTION The existence of high-rise buildings nowadays becomes a common sight in many cities, both in developed and developing countries. Various reasons stand behind its construction: the increasing demand of housing unit, limited land in urban areas, or by the reason of community prestige due to the existence of a highrise building considered to reflect economic and technological development of a region. Reinforced concrete (RC), as a popular building material is widely used due to some advantages such as low construction cost, simple treatment, and easily shaped for aesthetic purposes. In earthquake prone areas, it was necessary to ensure the safety of highrise building structures against the damage due to earthquakes, although rare of highrise buildings were damaged when earthquake struck, until the Mexico earthquake in 985. That earthquake destroyed many highrise buildings because of its characteristic of a long period which resonated with high-rise buildings that had the same characteristic of period. One of the methods to ensure the structural safety Researcher, Research Institute for Human Settlements (Puskim), Ministry of Public Works (PU), Indonesia. Chief Research Engineer, International Institute of Seismology and Earthquake Engineering (IISEE), Building Research Institute (BRI), Japan.

2 is to identify the dynamic parameters and monitor the behavior of building, to find out some tendency of the decreasing quality of structure that will lead to the fatal failure anytime when earthquake strikes. The big earthquake of magnitude M9. struck Japan on March, at 4:46 JST, the epicenter was at off-shore of Sanriku (38.3ºN, 4.86ºE), in 4 Km depth. The magnitude of the main shock was the largest in Japan up to the present (). According to the report of The Headquarter for Earthquake Research Promotion (), this event had a maximum seismic intensity 7 JMA observed in Kurihara City, Miyagi prefecture. The maximum aftershock was a M7.7 earthquake that occurred at 5:5 on March, as of April. There were more than 6 aftershocks of M6. or over. On April 7, there was a M7. earthquake with a seismic intensity 6 Upper observed in Miyagi prefecture. The aftershock area extends approximately 5km in an N-S direction. There is fear that large aftershocks will occur from now on, and there is a possibility that the area will be hit by strong shaking and high tsunami. The severe damages on buildings were mostly caused by the tsunami, thus concentrated along the eastern coast of Honshu Island (pacific coast), causing collapse or dragged away of buildings. The earthquake itself did not significantly affect structural damages of new buildings (designed by the building code after 98) or retrofitted buildings. Although only minor structural damages were found on some buildings, the successive earthquakes after the mainshock had led to fears of increasing levels of damage. The K-Tower and the S-Tower are the target RC high-rise buildings in this study, both located in Tokyo, Japan. The K-Tower is 9 m height which has 37 stories, three strong motion sensors were planted in this building, located on the first floor (F), middle floor (8F) and top floor (37F). The S-Tower is.99 m height which has 3 floors, two strong motion sensors were planted in the building i.e. on the first floor (F) and the top floor (3F), one sensor planted underground. The strong motion data records of the K-Tower to be analyzed in this study are 7, while that of the S-Tower are 6. Those buildings have regular shapes in vertical as well as in horizontal, the appearance and general plan view of the buildings can be seen briefly in Figure and. Figure. The K-Tower Figure. The S-Tower. OBJECTIVES AND METHOD The strong motion data of the Target RC high-rise buildings that recorded the successive earthquakes: before, during and after the mainshock of the Off the Pacific Coast of Tohoku Earthquake were used as input data in system identification to obtain dynamic parameters of the RC high-rise buildings in each earthquake event. The dynamic parameters as results of system identification were then used as input data in response analysis to observe the behavior of the structural model in the multy degree of freedom system (MDOF) and to determine the adequate damping type of the structural model under some earthquake motions. The health monitoring conducted as combination of system identification and response analysis to monitor the condition of the more specific part of the building. The results of those analysis stages were studied to draw some conclusions. Analysis of the strong motion data was performed by using the ViewWave software (developed by DR. T. Kashima, BRI), while the response analysis of the MDOF model was performed by using the MDOF OS software (developed by DR. T. Saito, BRI). 3. SYSTEM IDENTIFICATION The strong motion data records of the K-Tower and the S-Tower are analyzed in this chapter. The dynamic parameters that will be obtained are frequency and period which are identified by Fourier spectral ratio, and damping factor which is calculated by using half power method. The scatters of the

3 modal frequency of the K-Tower and S-Tower are shown in Figure 3 and 4 respectively, the red dashed line represent the mainshock of the Off the Pacific Coast of Tohoku earthquake. Frequency, F(Hz) Frequency, F(Hz) Data Number Figure 3. Frequency of the K-Tower The mainshock The mainshock Mode Mode Mode 3 Mode 4 Mode Mode Mode 3 Mode Data Number Figure 4. Frequency of the S-Tower All of modal dynamic parameters of the K-Tower and S-Tower were analyzed up to the 4 th mode and the average value before and after the mainshock were calculated, the shift of those parameters before and after the mainshock can be seen in Table. Building K-Tower S-Tower Table. The average of dynamic parameters before and after the mainshock Mode Before the mainshock After the mainshock Shift (%) F(Hz) T(sec) h(%) F(Hz) T(sec) h(%) F T h RESPONSE ANALYSIS Response analysis was conducted to the K-Tower for which there was structural data available. Two strong motion data will be selected among the successive earthquakes, one is the strong motion data before the mainshock that occurred on July 6, 7 (data no.3) which caused the maximum acceleration on the top floor 5.9 gal, the other is the mainshock of the Off the Pacific Coast of Tohoku earthquake (data no.) which caused the maximum acceleration on the top floor 98.3 gal, occurred on March,. The period as an analysis result from the data records is used to compare the periods that obtained from the response analysis of the MDOF model. There are 4 kinds of analysis result, each depends on the damping type assumed in the analysis, including: Proportional damping to initial stiffness, Rayleigh damping, modal damping, and proportional damping to nonlinear stiffness. The comparisons of periods before and during the mainshock are shown in Table.

4 Table. Comparison of period (unit: second) Prop. damping to Rayleigh Event Mode Record init. stiffness (h) damping (h) Before the mainshock During the mainshock Modal Damping (h3) Prop. damping to NL Stiffness (h4) %.974 %.974 %.974 % %.74 9%.74 9%.745 9% %.45 9%.45 9%.46 3% %.35 37%.35 36%.35 36% % 3. 9% 3. 9% 3.6 % %.75 7%.75 7% % %.4 %.4 %.4 % %.337 8%.353 3%.353 3% In Table, before the mainshock, all of the damping types give the same similarity in the st and nd mode, but in the 3 rd mode the Rayleigh and modal damping give the closest period, while in the 4th mode it is closed by the modal damping and the proportional damping to nonlinear stiffness. During the mainshock, the proportional damping to initial stiffness gives the closest period to that of the data record in the st mode, in the nd mode the proportional damping to initial stiffness and to nonlinear stiffness give the same similarity, in the 3 rd mode all of the damping types give the same similarity, while in the 4 th mode the Rayleigh damping gives the closest value. The comparisons of maximum acceleration before and during the mainshock are shown in Table 3. Before the mainshock, the Rayleigh and modal damping give the closest maximum acceleration to the data record on the 8 th floor, while the proportional damping give the closest maximum acceleration on the 37 th floor. During the mainshock, the proportional damping to nonlinear stiffness give the closest maximum acceleration to the data record on the 8 th floor, while on the 37 th floor the Rayleigh and proportional damping to nonlinear stiffness give the same similarity. Table 3. Comparison of maximum acceleration (unit: gal) Event Record Prop. damping to Rayleigh Modal Prop. damping to init. stiffness (h) damping (h) damping (h3) NL stiffness (h4) Before the % % %.78 95% mainshock % % % 4. 79% During the % 6.6 5% 7.9 % % mainshock % 6.6 4% 9. 6% % The structural performance under the strong motion before and during the mainshock are represented in maximum ductility and hysteresis curve as shown in Figure 5 and Crack a. Max. ductility b. Hysteresis curve a. Max. ductility b. Hysteresis curve Figure 5. The performance before the mainshock Figure 6. The performance during the mainshock 5. HEALTH MONITORING In the K-Tower, the strong motion sensors installed on the first floor (F), middle floor (8F) and top floor (37F), thus identification of suspected damage can only monitor the two parts of the building, which is the lower part ( st floor to 8 th floor) and upper part (8 th floor to 37 th floor), while the details of each floor cannot be done due to limited number of installed sensors.

5 The method is to observe the changes of frequency along the occurrence time of earthquakes (7 to ) and try to find out which part has been changed or dominantly changed by the earthquake events. By applying Fourier spectral ratio to both the lower part (8F/F) and the upper part (37F/8F), the frequency and period of those parts can be obtained. Illustration of the method represent in Figure 7. The scatters of frequency are shown in Figure 8, and the values are shown in Table 4. Data record at the 37 th floor Data record at the 8 th floor Data record at the st floor Figure 7. Health monitoring Frequency, F(Hz) The mainshock 37F/8F 8F/F Data Number Figure 8. The scatters of frequency in 37F/8F and 8F/F Table 4. Frequency of the K-Tower Before the Mainshock After the Mainshock Position Shift Min Max Average Min Max Average (%) 37F/F F/8F F/F Frequency shift before and during the mainshock is represented for the whole part (37F/F), the lower, and the upper part as in Figure 9. Mode shape in the st and the nd mode can be seen in Figure. Data no.3 represents the strong motion before the mainshock, while data no. represents the mainshock. Frequency(Hz) 3 37F/F 3 8F/F 3 37F/8F Figure 9. The shift of frequency Frequency(Hz) Frequency(Hz) Figure. Mode shape To evaluate the building condition according to the change of vibration characteristics i.e. frequency and mode shape, several cases were arranged to simulate different damage scenarios of structure and tried to find the similar characteristics between the monitored building and the simulated structural model. The frequency shift of the K-Tower is similar to that of the structural model in damage scenarios of case 4 (damage on the top and bottom) and case 5 (damage on the entire structure) where the frequency obtained in the smaller earthquake reduces when the structure is struck by the bigger earthquake. The mode shape of the K-Tower is similar to the graph of mode shape of structural model in damage scenario case 5, although there was a different tendency in the nd mode because of the damage level in the simulation was much higher than that in the observed building. 6. CONCLUSION The natural frequency of the RC high-rise buildings after the mainshock was decreased, comparing to that before the mainshock. The frequency shift of the K-Tower reached 8%, while that of the

6 S-Tower was 4%. The natural period of the K-Tower observed before the mainshock was.9 sec and increased after the mainshock to.3 sec, while that of the S-Tower observed before the mainshock was. sec and increased after the mainshock to.8 sec. The damping factor of the K-Tower before the mainshock was.9% and increased after the mainshock to 4.4%, while that of the S-Tower before the mainshock was 4.4% and increased after the mainshock to 5.%. The decrease of frequency indicates the stiffness degradation that could be caused by the structural damage. Modal damping and Rayleigh damping in the MDOF model before the mainshock, gave the closest parameters to the data record from the real building, but during the mainshock, the closest parameters was given by the proportional damping to initial stiffness and to nonlinear stiffness. The strong motion data before the mainshock did not affect the structural members, the maximum ductility was.6, hence no stiffness degradation occurred, but when it was struck by the mainshock, almost the entire story exceeded the cracking capacity except the 4 top stories, the maximum ductility was.4. Thus, the stiffness degradation was caused although that did not reach the yielding capacity, hence the structure reached the inelastic condition and supposed there was a minor damage occurred. The shift of frequency on the upper and the lower parts were 5% and 8% respectively. By conforming the vibration characteristics (frequency shift and mode shape) of the RC high-rise building with the damage simulation of the MDOF model under the El-Centro earthquake 94, the similar characteristics were found to the simulation of case 5 when the structure damage was on the entire story. The maximum ductility of the MDOF model under the El-Centro was.4 which occurred on the 34 th story. 7. ACTION PLAN The method in this study can be applied to other RC high-rise buildings to monitor the condition of the structure when experiencing successive earthquakes. The analysis only used the strong motion data on the building in horizontal direction by neglecting vertical direction and the data of the underground. It is recommended to consider the effect of vertical ground motion and soil-structure interaction to get more satisfactory results. The identification of the vibration characteristics to recognize the mode shape was only performed up to the nd mode, because the strong motion sensors were installed only on the 3 locations. It is recommended to install more strong motion sensors to obtain the vibration characteristics in the higher modes. The same method can be tried to be applied to other types of structures to identify the behavior of the different types of high-rise buildings. ACKNOWLEDGEMENT I would like to express my gratitude to Dr. K. Morita for useful advices and sharing the valuable data and information, and to Dr. T. Kashima to provide the useful software for processing the data wave. REFERENCES Aoyama, H.,, In A.S. Elnashai & P.J. Dowling (Eds.), London: Imperial college press. BRI (Building Research Institute), 7, K-Tower installation of strong motion seismograph. Chopra, A.K., 995, New Jersey: Prentice-hall. Jishin (Headquarters for Earthquake Research Promotion), 9 Kashima, T.,, 4 th European Conference on Earthquake Engineering. Koh, C.G., and Perry, M.J.,, Leiden: CRC press. Mari, J.L., Glangeaud, F., and Coppens, F., 999, Paris: Editions technip. Morita, K., -, Lecture notes on system identification in vibration analysis, IISEE/BRI. Shibata, A.,, Sendai: Tohoku university press. Web site: Jishin, Headquarters for Earthquake Research Promotion,